How to Make It
In a medium saucepan, combine the olive oil, thyme, bay leaf, fennel, carrot, coriander seeds, black and white peppercorns and half of the sliced onion. Add the tuna, submerging it in the oil. Attach a candy or deep-fry thermometer to the pan and warm the oil over moderate heat to 160°. Remove the saucepan from the heat and let the tuna cool at room temperature until slightly warm, about 30 minutes. Remove the tuna from the oil, break it into 1-inch pieces and transfer to a large bowl. Strain the poaching oil; reserve 1/4 cup of the oil and discard the solids and remaining oil.
Add the tomato slices and remaining onion to the bowl with the tuna. In a small bowl, whisk 1/2 cup of the reserved poaching oil with the sherry vinegar, oregano, lemon thyme, basil and mint. Pour the mixture over the tuna and tomatoes. Season with salt and pepper and toss gently.
Arrange the salad on a serving platter and top with any dressing left in the bowl. Drizzle the salad with the remaining 1/4 cup of reserved poaching oil and serve.